837d58e · typeable/aeson

git.iko.soy/typeable/aeson

Merge pull request #511 from andrewthad/better_example_fromjson

improve example FromJSON instance

commit837d58e50afbf369b08bccc774c188f02cadda0c authorAdam Bergmark <adam@bergmark.nl> dateMon 13 Feb 2017, 21:34 (10 years ago) parent71833a8 0ee9766

2 files changed+24−13

compare with master

unifiedsplit

Data/Aeson.hs+3−5 Data/Aeson/Types/FromJSON.hs+21−8

Data/Aeson.hs+3−5

@@ -274,11 +274,9 @@ eitherDecodeStrict' =

274274-- > {-# LANGUAGE OverloadedStrings #-}

275275-- >

276276-- > instance FromJSON Person where

277−-- > parseJSON (Object v) = Person <$>

278−-- > v .: "name" <*>

279−-- > v .: "age"

280−-- > -- A non-Object value is of the wrong type, so fail.

281−-- > parseJSON _ = empty

277+-- > parseJSON = withObject "Person" $ \v -> Person

278+-- > <$> v .: "name"

279+-- > <*> v .: "age"

282280--

283281-- We can now parse the JSON data like so:

284282--

Data/Aeson/Types/FromJSON.hs+21−8

@@ -288,7 +288,7 @@ genericLiftParseJSON opts pj pjl = fmap to1 . gParseJSON opts (From1Args pj pjl)

288288-- * 'typeMismatch' produces an informative message for cases when the

289289-- value encountered is not of the expected type

290290--

291−-- An example type and instance:

291+-- An example type and instance using 'typeMismatch':

292292--

293293-- @

294294-- \-- Allow ourselves to write 'Text' literals.

@@ -297,14 +297,27 @@ genericLiftParseJSON opts pj pjl = fmap to1 . gParseJSON opts (From1Args pj pjl)

297297-- data Coord = Coord { x :: Double, y :: Double }

298298--

299299-- instance FromJSON Coord where

300−-- parseJSON ('Object' v) = Coord '<$>'

301−-- v '.:' \"x\" '<*>'

302−-- v '.:' \"y\"

300+-- parseJSON ('Object' v) = Coord

301+-- '<$>' v '.:' \"x\"

302+-- '<*>' v '.:' \"y\"

303303--

304−-- \-- We do not expect a non-'Object' value here.

305−-- \-- We could use 'mzero' to fail, but 'typeMismatch'

306−-- \-- gives a much more informative error message.

307−-- parseJSON invalid = 'typeMismatch' \"Coord\" invalid

304+-- \-- We do not expect a non-'Object' value here.

305+-- \-- We could use 'mzero' to fail, but 'typeMismatch'

306+-- \-- gives a much more informative error message.

307+-- parseJSON invalid = 'typeMismatch' \"Coord\" invalid

308+-- @

309+--

310+-- For this common case of only being concerned with a single

311+-- type of JSON value, the functions @withObject@, @withNumber@, etc.

312+-- are provided. Their use is to be preferred when possible, since

313+-- they are more terse. Using @withObject@, we can rewrite the above instance

314+-- (assuming the same language extension and data type) as:

315+--

316+-- @

317+-- instance FromJSON Coord where

318+-- parseJSON = withObject \"Coord\" $ \v -> Coord

319+-- '<$>' v '.:' \"x\"

320+-- '<*>' v '.:' \"y\"

308321-- @

309322--

310323-- Instead of manually writing your 'FromJSON' instance, there are two options